im want to build a discharging circuit using arduino to discharge a 300mAh 8.4v NiMh cell. been doing some research on the web for weeks now and stumbled upon a constant current sink circuit using tl431. The circuit is on page 8 of the datasheet
http://www.100y.com.tw/pdf_file/38-ON-TL431,NCV431.pdf

before building the circuit i would like to get some suggestion from fellow arduinians. Do i have to use high wattage resistor for Rs? or is there any better or easier way to discharge a battery at 100mA??

thanks.it works like a charm
anyway,im also planning on implementing a discharger with my system so i found this constant current sink
http://www.eevblog.com/2010/08/01/eevblog-102-diy-constant-current-dummy-load-for-power-supply-and-battery-testing/
im planning to drain the battery at 100mA and then sending the data through serial comm which will later be analyzed by labview to produce a graph and the actual mAh of the battery.
I got the labview program running OK but cant find those mtp3055. Can anyone suggest other logic level n channel mosfet that is more common and can be applied to this constant current sink circuit?

sorry for the late reply. quite busy with other projects. anyway, measured the filtered voltage of the adapter using a 1000uF 16v electrolytic capacitor by placing it between the output of the adapter..the value is 12.4v. the same as the unfiltered voltage. its hard to get a 16v adapter here. most of them are either 12v or selectable adapter up to 12v

thanks for the info. im thinking of using the delta v method to stop the charging process or should i just stop at ~10.4v?
i only have 12v adapter around to power up the regulator. should i make a bridge circuit so that i can power it up directly from the wall outlet?
thinking about adding bridge circuit and another regulator to set the input voltage to ~16v

my wrong there,i draw it wrong in orcad but yes,in the real test circuit the 33ohm resistor is in series with the load. and i remove r2 and r3 from the circuit to make sure all the current goes to the battery and to remove all other variables. when i turn on the adapter and measure the voltage of the battery,it goes up to 9.5v and then i turn off the adapter because the battery is rated at 8.4v only (dont have any goggles around).
what is wrong?
or i shouldnt remove r2 and r3 from the circuit?
this is the test circuit but without r2 and r3 and the value of r1 is 33ohm

*will try to build the circuit as you suggested this weekend and hook it up to my arduino and re measure everything

referring to the datasheet, http://www.ti.com/lit/ds/symlink/lm117.pdf under the typical applcation note,theres an example of 50mA Constant Current Battery Charger. i think this is the basic circuit that peter hayles used in his circuit to construct the charger right? correct me if im wrong but i think my test circuit is correct. its just that the output voltage of the regulator are a bit higher than the voltage of the battery. its gives about 8.8v but the battery voltage is 8.4v so is it safe for me to charge the battery with this voltage?

dude,i doesnt get it why you said that this test circuit is wrong..is it wrong in term of application or in term of the value for the resistor?
i took your advice and change the value of the resistor to 33ohm so that the maximum current that can be drawn from the regulator is 1.25/33 = 0.0379A
then i connect the battery t0 the circuit and measure the current drawn by the battery.
its about 0.03A - 0.04A which is the same as when im using 1ohm resistor across adj and output pin.
my conclusion is that this battery are only capable of drawing 0.03-0.04A of currents only. am i right?
so no fast charging for this battery?

this is the basic circuit that i mean.the voltage that i measure is at vout. the total value of r2 and r3 is 1.8k those 2 resistor are to scale down the voltage so that it can be read by arduino. i increase the value of the resistor between output and adj to get a maximum current of 0.3A. the value of vout is 9.8v-ish..this is wayy to high to charge a 8.4 v battery right? since im using only small current to charge the battery,issit ok if i construct another voltage divider to decrease the voltage to about 8.4v? or any other suggestion? 1 have only 2 type of 5w resistor..one is 3.9ohm and another is 1 ohm..even putting both resistor in series does not decrease the value of vout

i constructed the circuit basic charging circuit using lm317,1ohm resistor and the diode to test the function of the charger before connecting it to my arduino. im using 300mAh 8.4v battery to test it and im powering the lm317 using adaptor rated at 12v 0.7A.since im using 1ohm 5W resistor on the output pin,the maximum current that can be produced should be 1.25A but since my adaptor are rated at 0.7A max,the circuit should produce 0.7A of current despite the value of resistor between the output and adj pin right?
but when i try to measure the current,it is around 0.04A only.the voltage are around 10v which is okay but the problem is the low current supplied to the battery. any ideas why?
is it because the high voltage of the output which is around 10v thus limiting the current supplied to the battery?

this is a little out of topic but im a little bit confused. Battery are charged  based on the current or voltage? or both? regarding this circuit,im using the voltage regulator to regulate the maximum current right? can this circuit be used to charge batteries of different voltage? i got confused because i bump into another circuit that controls both the voltage and current

*nevermind guys..i got it already..there is various method to charge a battery..constant voltage,constant current, taper,trickle, pulsed charging and etc. in this case its using pulsed charging right? thanks anyway to those who helped

PWMing allows you to control the charging rate. Since the output is either on or off, you cannot
vary the actual output current continuously, it's either 0 or 1.25 Amp, but you can control the
"average" output current. Eg, 10% duty-cycle gives average current = 0.125 Amp.

since it will only be in either on or off state, does that means that the battery will still be charged with 1.25A or it will be charged with the average value?

what is the purpose of that 1k resistor at adj pin? is it to reduce the current through the bjt when the base of the bjt are biased so that the high current from OUT doesnt fry the bjt?

im trying to come out with a project like this http://www.angelfire.com/electronic/hayles/charge1.html#fig3
but instead of using pic,i want to implement it using arduino but the problem is i cant understand how the charging current are controlled using the bjt and pwm.what i understand is by supplying pwm signal to the bjt,it will force the current to ground thus scaling down the current supplied to the battery.
any help would be much appreciated
*sorry for my bad english